Today, mobile electronic devices such as, for example, mobile telephones, personal digital assistants (“PDAs”), or tablets are usually equipped with one or more camera modules. These camera modules are used for very different purposes, e.g., for creating portrait recordings, reading barcodes or recording video sequences. Many of these compact camera modules have a low standard resolution of between 0.3 megapixels (“MP”, corresponding to VGA resolution) and 3 MP. For a few years now, use is increasingly made of high-resolution camera modules with 5 MP, 8 MP or 12 MP with rising market shares. All of these camera modules have a fixed focal length since a mechanism for adjusting the focal length would be complicated on account of the small installation depths and would be associated with high costs.
The fixed focal length represents a significant restriction for certain photographic tasks. By way of example, there is often the desire for a modifiable field angle when recording macroscopic details, when recording large structures, or when objects are far away. The known camera modules usually have a field angle of 75° (specified over the whole diagonal in the object space) and are provided with a front stop, usually on the object side in front of the first lens element, in rare cases with a stop directly after the first lens element.
In order to address the desire for a modifiable field angle, appropriate supplementary lenses are commercially available. In the case of wide-angle attachments or telephoto attachments, these supplementary lenses are designed as afocal supplementary lenses. This ensures that focused imaging is maintained at an infinite object distance. The necessary focusing can continue to be carried out using the camera module. Usually, afocal supplementary lenses are characterized by a magnification factor. Thus, a magnification factor of “0.6×” means that the focal length of the overall system including supplementary lens and camera module lens is reduced by a factor of 0.6 in relation to the focal length of the camera lens on its own. Hence, the corresponding field angle is increased. Conversely, a magnification factor of “2×” means that the focal length of the overall system including the supplementary lens and camera module lens is increased by a factor of 2 and the field angle is correspondingly reduced.
Commercially available supplementary lenses suffer from deterioration of the imaging performance, for example, at the edge of the image field; this is already considered bothersome in the case of an unmagnified observation. The deterioration of the imaging performance affects, in particular, the so-called asymmetry aberrations, i.e., coma, distortion, and astigmatism. Added thereto are deteriorations in the imaging as a result of a decentration or tilt of the supplementary optical unit in relation to the optical axis of the camera module.
A further problem is posed by the supplementary optical units, on account of their inherent design, having to become larger as an improved imaging performance is sought. If the known supplementary optical units go beyond a simple convex converging lens element and have an afocal design, a lens element with positive refractive power is combined with a lens element with negative refractive power. If higher imaging performances, e.g., a magnification factor of “4×”, are sought, a resulting diameter for a suitable supplementary optical unit can be calculated to be 65 mm without housing. Although these dimensions can be realized in theory, they have unsatisfactory handling as a consequence in practice. Firstly, the supplementary optical unit then dominates the combination of mobile electronic device and supplementary optical unit on account of its sheer size, and so the advantage of such an additional optical unit over a dedicated camera is lost in this respect.
Secondly, two or more camera modules are increasingly installed next to one another in the aforementioned mobile electronic devices. By way of example, these are used for stereo recordings or distance measurements, or have two different focal lengths which are used for different purposes. If two camera modules are used next to one another, distances of between 9 mm and 20 mm are common, for example. If the supplementary optical unit had the aforementioned dimensions, one of the two camera modules would be covered when the supplementary optical unit is used.